ed or naturally selected.
And here the importance of the principle of benefit being derived from
divergence of character comes in; for this will generally lead to the most
different or divergent variations (represented by the outer dotted lines)
being preserved and accumulated by natural selection. When a dotted line
reaches one of the horizontal lines, and is there marked by a small
numbered letter, a sufficient amount of variation is supposed to have been
accumulated to have formed a fairly well-marked variety, such as would be
thought worthy of record in a systematic work.

[Illustration]

The intervals between the horizontal lines in the diagram, may represent
each a thousand generations; but it would have been better if each had
represented ten thousand generations. After a thousand generations, species
(A) is supposed to have produced two fairly well-marked varieties, namely
a^1 and m^1. These two varieties will generally continue to be exposed to
the same conditions which made their parents variable, {118} and the
tendency to variability is in itself hereditary, consequently they will
tend to vary, and generally to vary in nearly the same manner as their
parents varied. Moreover, these two varieties, being only slightly modified
forms, will tend to inherit those advantages which made their parent (A)
more numerous than most of the other inhabitants of the same country; they
will likewise partake of those more general advantages which made the genus
to which the parent-species belonged, a large genus in its own country. And
these circumstances we know to be favourable to the production of new
varieties.

If, then, these two varieties be variable, the most divergent of their
variations will generally be preserved during the next thousand
generations. And after this interval, variety a^1 is supposed in the
diagram to have produced variety a^2, which will, owing to the principle of
divergence, differ more from (A) than did variety a^1. Variety m^1 is
supposed to have produced two varieties, namely m^2 and s^2, differing from
each other, and more considerably from their common parent (A). We may
continue the process by similar steps for any length of time; some of the
varieties, after each thousand generations, producing only a single
variety, but in a more and more modified condition, some producing two or
three varieties, and some failing to produce any. Thus the varieties or
modified descendants, proceeding from